Appropriate management of physical and mental fatigue of laborers in diversified labor environments is indispensable in modern society not only to protect the laborer's own health but also to prevent accidents which can affect the laborer involved and other people, possibly the whole society. Hitherto, physical fatigue was evaluated relatively easily in reference to some external indices such as muscular weakness, whereas mental fatigue was evaluated mostly by carrying out surveys or the like. Therefore, it has always been difficult to exclude arbitrariness from mental fatigue evaluation.
A change in the threshold (may also be referred to as a flicker value) of the frequency at which the user can perceive the flickering of a flicker light is a known index of mental fatigue. This index is based on a decrease in the activity of the entire cerebral cortex, which is a major symptom of mental fatigue. The flicker of light flickering at a high speed is generally not perceived by a user; instead, the user perceives it as a constant illumination. As the flicker frequency is gradually decreased from a high value to a low value, the user perceives the flicker at a certain point. The frequency at which the user perceives the flicker is called a flicker frequency threshold or a flicker value. There is a known phenomenon in which the user becomes incapable of perceiving the flicker at a certain threshold frequency due to the accumulation of mental fatigue resulting from labor, even though the user was able to perceive it when he/she was in a general healthy condition (when the user had no mental fatigue). More specifically, the threshold frequency at which the user perceives the flicker, i.e., the flicker value, decreases due to mental fatigue.
Though they are not widely known, there are methods of evaluating mental activity, such as the degree of fatigue, by measuring the physiological reaction of the eyes, and a method of fatigue recovery by exposing the eyes to optical stimulation (see the following Patent Documents 1 to 3). However, a change in the activity of the entire cerebral cortex due to mental fatigue induced by various labor has been evaluated mostly by making the user subjectively determine the flicker perception threshold among variable flicker frequencies with a push-button operation or the like (for example, see the following Patent Documents 4 to 6).
[Citation List]
[Patent Document 1] Japanese Unexamined Patent Publication No. 7-255669
[Patent Document 2] Japanese Unexamined Patent Publication No. 2002-253509
[Patent Document 3] Japanese Unexamined Patent Publication No. 2005-279053
[Patent Document 4] Japanese Unexamined Patent Publication No. 2001-309887
[Patent Document 5] Japanese Unexamined Patent Publication No. 2001-218756
[Patent Document 6] Japanese Unexamined Patent Publication No. 2003-70773